There is no admission that the background art disclosed in this section legally constitutes prior art.
Photovoltaic cells are devices that convert light energy into electrical energy. Typical photovoltaic cells include a substrate layer for mounting the cell and two ohmic contacts or electrode layers for passing current to an external electrical circuit. The cell also includes an active semiconductor junction, usually comprised of two or three semiconductor layers in series. The two layer type of semiconductor cell consists of an n-type layer and a p-type layer, and the three layer type includes an intrinsic (i-type) layer positioned between the n-type layer and the p-type layer for absorption of light radiation. The photovoltaic cells operate by having readily excitable electrons that can be energized by solar energy to higher energy levels, thereby creating positively charged holes and negatively charged electrons at the interface of various semiconductor layers. The creation of these positive and negative charge carriers applies a net voltage across the two electrode layers in the photovoltaic cell, establishing a current of electricity.
Photovoltaic cells have been produced using a variety of materials for the various functional layers of the cell. In particular, semiconductor layers of alloys using cadmium, tellurium, sulfur, indium, gallium, and even iron are known in the art. For example, iron pyrite (iron persulfide, FeS2) has been tested as a material thought to be suitable for semiconductor layers due, in large part, to its crystal structure and the predicted optical and semiconducting properties associated with that structure. The predicted semiconducting and optical properties of FeS2 are similar to other well-known semiconductor materials, which has directed research efforts toward eliminating observed characteristics of the material that differ from the predicted material properties. For example, efforts have been made to reduce the free-carrier concentrations of the material to lower levels consistent with those of semiconductors. FeS2 exhibits an indirect band gap of 0.95 eV and an absorption coefficient exceeding 105 cm−1 for photon energies above 1.3 eV. Previous attempts at creating FeS2-based PV devices have performed poorly compared with devices based on other absorber materials (e.g. CdTe, CIGS, CZTS, perovskites such as methylammonium lead iodide, and PbS quantum dots). These previous efforts were only able to produce a 2.8% efficient photo-electrochemical cell based on single crystal FeS2 electrodes using an iodide/tri-iodide (I—/I3—) redox couple. This poor efficiency for FeS2-based solar cells is based, in part, on challenges presented by high defect densities, likely the result of poor phase and stoichiometry control. Specifically, phase purity is a concern for effective FeS2 photovoltaic devices because the different iron sulfide phases of FexSy exhibit a wide range of optoelectronic properties.
The ohmic contacts of photovoltaic cells are often configured as front and back contacts. The front electrical contact is a transparent or semi-transparent layer that is electrically conductive and permits light energy to pass through to the semiconductor layers below. The back contact is also electrically conductive but is not necessarily a transparent layer. Back contacts are known to include materials containing copper, gold, zinc, aluminum, and graphite, for example. These materials, however, may be adversely reactive with adjacent semiconductor layers, may pose manufacturing or durability concerns, or may be expensive. Thus, it would be desirable to provide a material for a back contact layer that exhibits suitable electronic properties, elemental abundance, and forms a low-barrier interface allowing positively-charged holes to transfer out of the semiconductor layer, such as the CdTe layer, to the back metal electrode. In particular, it would be desirable to produce a photovoltaic cell formed with a back contact layer from abundant and inexpensive materials, such as a FeS2-based back contact.